In general, a pressure detection device is known which is fitted to an engine and which uses a piezoelectric element or the like as a sensor to detect a combustion pressure within a combustion chamber. Since the sensor outputs a signal obtained by differentiating a pressure, the pressure detection device includes a processing circuit which converts a detection signal from the sensor into voltage similar to a variation in the pressure by integration with an integrator circuit.
Conventionally, as the processing circuit included in such a pressure detection device, a signal processing device of a piezoelectric sensor disclosed in patent literature 1 is known. FIG. 11 shows a schematic diagram of an integrator circuit in a pressure detection device which is disclosed in patent literature 1 and which detects the combustion pressure of an engine. In the integrator circuit 100 shown in FIG. 11, one terminal of a combustion pressure sensor 101 is connected through a capacitor 102 and a resistor 103 to the inverting input terminal of an operational amplifier 104, and the other terminal of the combustion pressure sensor 101 is connected to the GND of the circuit. On the other hand, the non-inverting input terminal of the operational amplifier 104 is connected to a reference power supply 105 to feed a reference voltage Vr. Between the inverting input terminal and the output terminal of the operational amplifier 104, a parallel circuit of a charge capacitor 106 and a discharge resistor 107 with a high resistance value is connected.
FIG. 12 is a waveform diagram illustrating the operation of the integrator circuit 100, the horizontal axis represents time t, the vertical axis of FIG. 12(a) represents a charge signal Qi from the combustion pressure sensor 101 and the vertical axis of FIG. 12(b) represents the output signal Vout of the integrator circuit 100. The combustion pressure sensor 101 detects a pressure to generate the charge signal Qi of a differentiated waveform shown in FIG. 12(a) at a predetermined period T01. Then, the charge signal Qi is fed to the integrator circuit 100. The integrator circuit 100 is operated using the reference voltage Vr from the reference power supply 105 as a reference, and the charge signal Qi is converted by integration into a voltage. In this way, at the output terminal of the operational amplifier 104, the output signal Vout of a voltage waveform similar to a variation in the pressure shown in FIG. 12(b) is obtained. When the charge signal Qi is transferred to the negative side, the output signal Vout is changed according to an ascending curve whereas when the charge signal Qi is transferred to the positive side, the output signal Vout is changed according to a descending curve. As described above, since the integrator circuit 100 integrates and outputs the charge signal Qi of the differentiated waveform, the voltage waveform of the output signal Vout is a waveform similar to a variation in the pressure detected by the combustion pressure sensor 101, with the result that it is possible to detect the combustion pressure.